System and method for cervical midline fixation

ABSTRACT

Devices and methods for enhancing the effectiveness of spinal stabilization, and particularly that of cervical spinal stabilization, are provided herein. More specifically, methods and systems are disclosed for effectively positioning occipital plates and spinal fixation assemblies within target vertebrae, while also reducing any associated patient trauma (e.g., muscle stripping, tissue damage, etc.). The systems and methods can utilize trans-lamina delivery of the spinal fixation assemblies to allow for the positioning of the fixation elements along the midline of the patient&#39;s spine.

FIELD OF USE

The present disclosure relates to devices and methods for use in variousspinal fixation procedures, in particular to devices and methods for usein cervical stabilization procedures.

BACKGROUND

Stabilization of the spine is often required to correct for trauma,tumor, or degenerative pathologies. Current methods of treatmentgenerally involve the use of a spinal fixation element, such as arelatively rigid fixation rod, that is coupled to adjacent vertebrae byattaching the fixation element to various anchoring devices, such asplates, hooks, bolts, wires, or screws. Spinal stabilization systems,which frequently include two fixation elements disposed on oppositesides of the midline of the spine, hold the vertebrae in a desiredspatial relationship, until healing or spinal fusion has taken place, orfor some longer period of time.

Due to the intricacies of working in the proximity of the spinal column,such stabilization procedures can result in significant trauma. Forexample, such procedures typically require that the anchoring devices beimplanted into a lateral mass of a target vertebra. In light of thistrajectory, significant amounts of muscle and tissue must be strippedfrom the treatment site due to the relatively large distance between thelateral mass entry point and the midline of the spinal column. Further,any slight miscalculation in the delivery trajectory can result inpenetration of a distal portion of the anchoring device (e.g., a pointedtip) into the spinal canal, thereby causing significant injury. As afurther disadvantage, the limited bone mass and/or bone densitytypically found in the lateral mass of a vertebra significantly limitsthe ability of the vertebra to effectively engage the anchoring devices.

Thus, there remains a need for methods and systems capable of securelypositioning fixation assemblies within target vertebrae while alsominimizing the risk of injury and associated patient trauma.

When such surgery is performed in the cervical spine, the fixationelements are typically molded according to the anatomy of the skull andthe cervical spine, and attached to a fixation plate that is implantedin the occiput. Typically, the occipital plate (e.g., a T-shaped orY-shaped plate) is positioned along the midline of a patient's occiputsuch that a single fixation plate can engage spinal fixation elementsthat run on either side of the midline.

Although each region of the spine presents unique clinical challenges,posterior fixation of the cervical spine is particularly challengingbecause the anatomy of the cervical spine makes it a technicallydifficult area to instrument. Specifically, several vital neural andvascular structures, including the vertebral arteries, nerve roots, andspinal cord must be avoided during surgery.

Accordingly, there remains a need for improved spinal fixation devicesand methods of improving and/or optimizing cervical stabilizationprocedures.

SUMMARY

Devices and methods for enhancing the effectiveness of spinalstabilization, and particularly that of cervical spinal stabilization,are provided herein. More specifically, methods and systems aredisclosed for effectively positioning occipital plates and spinalfixation assemblies within target vertebrae, while also reducing anyassociated patient trauma (e.g., muscle stripping, tissue damage, etc.).As described below, the systems and methods can utilize trans-laminadelivery of the spinal fixation assemblies to allow for the positioningof the fixation elements along the midline of the patient's spine.

Various aspects of an implantable assembly are provided herein. In afirst aspect, an implantable assembly is provided which includes a platehaving a bone contacting surface configured to be positioned on theocciput and an opposed surface for seating a spinal fixation element.The plate can have at least one opening extending through the bonecontacting surface and the opposed surface for receiving an anchorelement. Further, a pair of mating flanges can extend from the opposedsurface and can be configured to receive a floating nut such that thefloating nut and the opposed surface are configured to secure the spinalfixation element therebetween along a longitudinal axis of the plate. Inone aspect, the longitudinal axis is configured to be positioned over amidline of the spine.

The bone contacting surface and the opposed surface can have a varietyof configurations. For example, the opposed surface can include a groovefor seating the spinal fixation element. In one aspect, the at least oneopening can extend through the groove along the longitudinal axis of theplate. In another aspect, the at least one opening extends through theplate adjacent the groove, and can be, for example, disposed lateral tothe longitudinal axis.

The floating nut can also have a variety of configurations. For example,the floating nut can have an inferior surface configured to seat thespinal fixation element. In one aspect, the floating nut can includelateral projections configured to slidably engage the flanges. By way ofexample, the lateral projections can dovetail with the flanges. Thefloating nut can also include a bore formed therethrough for receiving alocking element. The bore can, for example, extend through the floatingnut substantially perpendicular to the longitudinal axis. In one aspect,the locking element can be configured to secure the floating nut to thespinal fixation element. In such an embodiment, engagement of thelocking element with the spinal fixation element can prevent movement ofthe floating nut along the longitudinal axis relative to the plate.Further, in one aspect, the plate has at least two openings, and thefloating nut can be positioned in a plane above a plane on which the twoopenings are disposed.

In another exemplary embodiment, a spinal fixation assembly is providedwhich includes a housing having an anterior base and a pair of armsextending posteriorly therefrom. The pair of arms define a slottherebetween that is configured to seat a spinal fixation element. Theslot extends along a longitudinal axis of the housing and, in oneaspect, is configured to be aligned with a midline of the spine. Thehousing can also have a central axis perpendicular to the longitudinalaxis and extending through the base and the slot. An anchor-receivingopening, formed in at least one arm, extends between the slot and a bonecontacting surface of the at least one arm adjacent the base. The atleast one anchor-receiving opening is offset from the central axis andis configured such that at least one anchor member disposed therethroughextends inferiorly and laterally away from the housing at an acute anglerelative to the central axis. For example, the anchor-receiving openingcan be angled relative to the central axis to permit the anchor memberto be implanted within the vertebra in a trans-lamina orientation.

In one aspect, the anchor-receiving opening can be defined by aninternal surface of the housing that can, for example, be configured toengage a head of the at least one anchor member. In one aspect, theinternal surface of the housing can be substantially spherical to allowfor polyaxial movement of the at least one anchor member. The internalsurface of the housing can also be configured to seat the head of the atleast one anchor member offset from the central axis.

In one aspect, the base can also include a bone contacting surfaceadjacent the anchor-receiving opening. The bone contacting surfaces ofthe at least one arm and base can be configured to sit on the lamina.

In one embodiment, first and second arms of the housing can disposed onopposed sides of the central axis relative to one another. The secondarm can include a window formed therethrough that is configured toprovide access for a driver for manipulating an anchor member disposedthrough the anchor member opening in the first arm. In one embodiment,each of the first and second arms has an anchor-receiving opening. Byway of example, a first anchor-receiving opening in the first arm can beconfigured to receive a first anchor member and a secondanchor-receiving opening in the second arm can be configured to receivea second anchor member such that the first and second anchor membersextend laterally away from the housing in different directions. In oneaspect, the first and second anchor-receiving openings can be configuredto receive the first and second anchors when the anchors arepre-installed in a vertebra.

In another aspect, a spinal fixation system is provided which includes ahousing assembly, a spinal fixation element, and at least one anchormember. The housing assembly has a base and a pair of arms extendingtherefrom. The pair of arms define a slot therebetween that extendsalong a longitudinal axis of the housing and is configured to be alignedwith a midline of the spine. The housing also includes a central axisthat is perpendicular to the longitudinal axis and that extends throughthe base portion and the slot. As mentioned above, the system alsoincludes a spinal fixation element that is configured to be disposed inthe slot such that the spinal fixation element extends along a midlineof the subject's spine. Further, the at least one anchor member isconfigured to be disposed through an anchor-receiving opening extendingthrough at least one arm between the slot and a bone contacting surfaceof the at least one arm. The anchor member is offset from the centralaxis and extends away from the housing at an acute angle relative to thecentral axis.

In one embodiment, the spinal fixation element can be a rod. The rod canhave various configurations, for example, of an irregular or rectangularcross-section. In one aspect, a fin can be coupled to the final fixationelement. The fin can include one or more holes to which tissue can beattached. The anchor member can also have a variety of configurations.For example, in one embodiment, the anchor member can be a screwconfigured to be imbedded in a lamina. Alternatively, for example, theanchor member can be a hook configured to be hooked onto a lamina.

The system can also include a locking element configured to mate withthe arms to secure the spinal fixation element within the slot. In oneaspect, the inner surface of the arms can have threads for engaging thelocking element.

Various aspects of a method of providing spinal stabilization are alsodisclosed herein. In one such aspect, the method includes fixing anoccipital plate to the occiput of a subject with an anchor element andmounting a spinal fixation upon the occipital plate such that the spinalfixation element is positioned over a midline defined by the spinalcolumn of a subject. The spinal fixation element can be secured to theoccipital plate with a floating nut such that the spinal fixationelement is positioned between the occipital plate and the floating nut.In one aspect, the floating nut can be slid along the spinal fixationelement mounted on the occipital plate to position the floating nutbetween the spinal fixation element and a portion of the occipitalplate.

The occipital plate can be fixed to the occiput in a variety of ways. Byway of example, the occipital plate can be fixed to the occiput byinserting an anchor element into the occiput through at least oneopening extending through the plate. In one embodiment, the at least oneanchor element is inserted into the occiput on the midline. Further, thespinal fixation element can be positioned over the at least one anchorelement. In another aspect, the at least one anchor element can beinserted into the occiput offset from the midline.

In another exemplary embodiment, a method of providing spinalstabilization is provided which includes positioning a first fixationassembly upon a first vertebra. The first fixation assembly includes aproximal housing having a base and a pair of arms and can be positionedsuch that a slot extending between the arms and along a longitudinalaxis of the first fixation assembly is aligned with a midline of thespine. An anchor member, seated in the first fixation assembly, canextend away from the housing at an acute angle relative to a centralaxis that is generally perpendicular to the longitudinal axis. Themethod can also include positioning a second fixation assembly within asecond vertebra such that a slot of the second fixation assembly isaligned with the slot of the first fixation assembly. A spinal fixationelement can be positioned within the slots of the first and secondfixation assemblies such that the spinal fixation element extends alonga midline of the spine. Further, the spinal fixation element can besecured within the slots of the first and second fixation assemblies.

The first fixation assembly, which can be the same or different than thesecond fixation assembly, can be secured to the lamina in a trans-laminaorientation during the step of positioning a first fixation assemblyupon a first vertebra. In such a method, the first fixation assembly canbe secured to the first vertebra with a single anchor member. In oneaspect, the anchor member can be seated in the fixation assembly beforebeing secured to the lamina. In another embodiment, the first fixationassembly can be secured to the first vertebra with two anchor membersimplanted within the lamina on opposed sides of the midline. The anchormembers can be seated in the fixation assembly after the anchor membersare implanted in the lamina.

These and other aspects of the presently disclosed methods and systemsare detailed below.

BRIEF DESCRIPTION OF THE DRAWINGS

The presently disclosed embodiments will be more fully understood fromthe following detailed description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a posterior view of an exemplary embodiment of a spinalfixation system engaged at desired anatomical locations;

FIG. 2 is a posterior view of an exemplary embodiment of an occipitalplate;

FIG. 3 is a posterior view of the occipital plate of FIG. 2, showing anexemplary embodiment of a floating nut engaged therewith;

FIG. 4 is an inferior view of the occipital plate and floating nut ofFIG. 3;

FIG. 5 is a perspective view of the floating nut of FIG. 3;

FIG. 6 is a posterior view of another exemplary embodiment of anoccipital plate, showing a floating nut engaged therewith;

FIG. 7 is an anterior view of the occipital plate and floating nut ofFIG. 6;

FIG. 8 is an anterior view of the occipital plate and floating nut ofFIG. 6;

FIG. 9 is a perspective view of an exemplary embodiment of a housing ofa spinal fixation assembly;

FIG. 10 is a perspective view of the housing of FIG. 9 having a singleanchor member disposed therein;

FIG. 11 is a superior view of the housing of FIG. 9 having a singleanchor member positioned within the lamina of a vertebra, whereinportions of the vertebra have been truncated prior to securing theassembly thereto;

FIG. 12 is a perspective view of another exemplary embodiment of ahousing of a spinal fixation assembly;

FIG. 13 is a posterior view of the housing of FIG. 12 having two anchormembers disposed therein;

FIG. 14 is a perspective view of the housing of FIG. 12 having twoanchor members disposed therein and a spinal fixation element coupledthereto;

FIG. 15 is a superior view of the housing of FIG. 12 having two anchormembers positioned within the lamina of a vertebra, wherein portions ofthe vertebra have been truncated prior to securing the assembly thereto;

DETAILED DESCRIPTION

Certain exemplary embodiments will now be described to provide anoverall understanding of the principles of the structure, function,manufacture, and use of the systems and methods disclosed herein. One ormore examples of these embodiments are illustrated in the accompanyingdrawings. Those skilled in the art will understand that the systems andmethods described herein and illustrated in the accompanying drawingsare non-limiting exemplary embodiments and that the scope of the presentdisclosure is defined solely by the claims. The features illustrated ordescribed in connection with one exemplary embodiment may be combinedwith the features of other embodiments. Such modifications andvariations are intended to be included within the scope of the presentdisclosure.

Devices, systems, and methods for optimizing various cervicalstabilization procedures are described herein. The devices describedherein can have a variety of configurations but are generally designedto allow a surgeon to position occipital plates and spinal fixationassemblies such that the spinal fixation element extends over a midlineof patient's spine, thereby reducing the risk of trauma (e.g., musclestripping, tissue damage, etc.) associated with prior posterior fixationprocedures of the cervical spine. Additionally, the systems and methodsdescribed herein can also utilize trans-lamina delivery of the spinalfixation assemblies, thereby enabling more secure fixation through theuse of larger (e.g., longer and/or wider) fixation assemblies relativeto those of prior posterior fixation procedures, which engage thevertebra at locations more distant the midline (e.g., the lateral mass).As a further advantage, the delivery trajectory enabled by suchtrans-lamina delivery and positioning reduces the potential forinadvertent damage to the spine and/or surrounding areas because theassemblies can be angled away from the patient's spinal canal duringdelivery.

As indicated above, traditional spinal stabilization techniquestypically require a first plurality of fixation assemblies (e.g., boneanchors coupled to a receiving head) engaged to a plurality of vertebraealong one side of the midline of a patient's spine, and a secondplurality of fixation assemblies engaged to vertebrae along an oppositeside of the midline. Once the fixation assemblies are secured to thevertebrae, a first rod is engaged to the first plurality of fixationassemblies, and a second rod is engaged to the second plurality offixation assemblies. Next, a superior portion of each rod is engaged toan occipital plate such that the fixation elements extend from thecervical vertebrae to the occiput lateral to the midline. Generally, asingle occipital plate spanning the midline is used to allow both thefirst and second rods to engage the same occipital plate.

In contrast to unilateral or bilateral stabilization methods and systemsin which the spinal fixation element(s) extend along the spine lateralto the midline of the patient's spine, FIG. 1 shows an exemplaryembodiment of a cervical stabilization system 100 according to theteachings herein. The system 100 generally includes an occipital plate120, first and second fixation assemblies 160, 160′, and a spinalfixation element 110 extending therebetween. As shown, the occipitalplate 120 can be engaged to the patient's occipital bone 2 on themidline (M.L.) of the patient's spine. As will be discussed in detailbelow, the occipital plate 120 can have a variety of configurations, butgenerally includes a bone contacting surface, an opposed surface 124 forseating the spinal fixation element 110, and at least one opening 126extending therethrough. As shown in FIG. 1, the opening 126 can beconfigured to receive a bone screw 128 or any other type of suitableanchoring device so as to anchor the occipital plate 120 to theunderlying occipital bone 2.

As shown in FIG. 1, the occipital plate 120 is generally configured tosecurely engage a spinal fixation element 110 disposed on the midline ofa patient's spine. For example, the occipital plate 120 can beassociated with a floating nut 140 that is configured to secure thespinal fixation element 110 between the floating nut 140 and the opposedsurface 124 of the occipital plate 120 as will be discussed in detailbelow. A locking element 142, described further below, can be disposedwithin the floating nut 140 to securely couple the floating nut 140 tothe spinal fixation element 110.

In addition to the occipital plate 120, the system 100 can also includea variety of spinal fixation assemblies that are generally configured tosecurely engage the vertebrae and provide a seat for the spinal fixationelement 110 extending inferiorly from the occipital plate 120. As willbe appreciated by a person skilled in the art, a variety of prior artspinal fixation assemblies modified in light of the teachings herein canbe used in conjunction with the occipital plate 120 to position thespinal fixation element 110 on the midline of the spine.

Now with specific reference to the exemplary embodiments of spinalfixation assemblies 160, 160′ depicted in FIG. 1, the spinal fixationassemblies 160, 160′ include a housing having a pair of arms 164extending posteriorly therefrom and disposed on opposed sides of themidline when implanted in the vertebrae. The arms 164 define a slot 166therebetween that is configured to seat the spinal fixation element 110along the midline of the spine. As will be discussed in detail below,the housing can have a central axis (perpendicular to the midline ofFIG. 1) and an anchor-receiving opening offset from the central axiswhich extends through at least one of the arms 164. The anchor-receivingopening can be configured such that an anchor member disposedtherethrough extends anteriorly and laterally away from the housing atan acute angle relative to the central axis. Thus, for example, theanchor member 168 extending from the superior fixation assembly 160 canbe implanted in the lamina 6 of the vertebra 4 on one side of themidline, as shown in FIG. 1. Similarly, with respect to the inferiorfixation assembly 160′, the two anchor members 168′ extend laterallyaway from the housing in different directions and are implanted in thelaminae 6′ of the vertebra 4′ on opposite sides of the midline.

As will be discussed in detail below, various embodiments of the methodfor implanting the system 100 can include modifying or truncatingvarious portions of the target vertebrae 4, 4′ so as to further optimizethe procedure and/or provide a desired clinical outcome (e.g.,decompression to alleviate pressure on the spinal cord). Briefly, asshown in FIG. 1, the spinous process of the vertebra 4′ has beenremoved, while the spinous process and one of the laminae (to the leftin FIG. 1) has been removed from the vertebra 4. By removing theseportions of the target vertebrae, the spinal fixation assemblies 160,160′ can access an optimal entry point of the vertebral bone and bepositioned on the midline of the patient's spine.

As indicated above, the slots 166 of the spinal fixation assemblies 160,160′ can be aligned on the midline of the spine such that the slots 166are configured to receive a spinal fixation element 110, such as a rod,extending from the occipital plate X along the midline. As shown in FIG.1, the system 100 can further include a locking element 170 (e.g., setscrew) that is configured to mate with the arms 164 of each of thespinal fixation assemblies 160, 160′ to secure the spinal fixationelement 110 within the slots 166.

One skilled in the art will appreciate that the occipital plate 120 andspinal fixation assemblies 160, 160′ can be configured to receive avariety of fixation elements. Suitable spinal fixation elements for usewith the present invention include, by way of non-limiting examples,rods, tethers, cables, plates, etc. The spinal fixation elements canhave a variety of configurations, and, by way of non-limiting example,can be rigid, semi-rigid, bendable, flexible, etc. As will beappreciated by a person skilled in the art, the spinal fixation elementscan include additional features which improve the integration of thesystem 100 within the patient's body. For example, in one embodiment,the spinal fixation element 110 can additionally include a fin to whichsoft tissue can be attached to promote integration and post-surgicalrecovery, as will be discussed in detail below.

In the exemplary embodiment illustrated in FIG. 1, the spinal fixationelement 110 is an elongate rod. While the rod 110 can be substantiallystraight, in the illustrated embodiment, the rod 110 is bent or curved(not shown) to allow the rod to extend from the cervical vertebrae tothe occipital plate 120 fixed on the occiput 2. The bend or curve cantake any shape, but it can be preferable for the rod 110 to becomplementary to a curve of the spine. Thus, the shape of the rod 110can be substantially similar to a natural curve of the spine along themidline (M.L.). For example, the rod 110 can be curved to extend fromthe spinous process of one vertebra to the spinous process of anadjacent vertebra, while maintaining a close association with thecontours of the spinal column therebetween. In some instances, the curveof the rod 110 can be pre-determined. In other instances, the rod 110can include some flexibility to allow the rod 110 to be shaped in accordwith its implant location. In even other instances, the rod 110 can befully bendable so it can be formed into any desired shape along itslength.

The rod 110 can also have a variety of cross-sections. For example, therod 110 can have a circular cross-section. Alternatively, rods for useon the midline of the spine can also be shaped so as to provideincreased torsional stability. For example, in one embodiment, the rod110 can have an irregular and/or rectangular cross-section.

In addition to the various embodiments of the systems and devices forspinal stabilization described above, methods for providing spinalstabilization are also described herein. For example, with reference toFIG. 1, a method for cervical midline fixation can include fixing theoccipital plate 120 to the occiput 2 of a subject, securing a firstspinal fixation assembly 160 to a first vertebra 4, and securing asecond fixation assembly 160′ to a second vertebra 4′. In the depictedembodiment, the occipital plate 120 and first and second fixationassemblies 160, 160′ are positioned so as to be aligned on the midlineof the patient's spine. A spinal fixation element 110 can then besecured to the occipital plate 120 and the first and second fixationassemblies 160, 160′ such that the spinal fixation element extendstherebetween on the midline of the patient's spine.

With reference now to FIGS. 2-4, one exemplary embodiment of anoccipital plate 220 is shown in more detail. As indicated above, theoccipital plate 220 can have a variety of configurations, but isgenerally configured to be fixed to a patient's occiput for securing aspinal fixation element thereto. As shown in FIG. 2, the occipital plate220 can be in the form of a generally elongate member that defines alongitudinal axis (L) extending between inferior and superior ends 230a,b thereof. Though the occipital plate 220 can be fixed to any locationof the occiput during surgery, in one embodiment, the longitudinal axisis configured to be aligned with a midline of the patient's spine whenfixed to the occiput. The shape of the occipital plate 220 can vary, andwill typically depend on the nature of the procedure and/or thepatient's anatomy. For example, in one embodiment, the occipital platecan have a substantially constant width from the inferior end 230 a tothe superior end 230 b. In the embodiment depicted in FIG. 2, however,the width of the occipital plate 220 tapers at the superior end 230 b.Additionally, the inferior and superior ends 230 a,b can be rounded (oreven convex) so as to avoid the risk of damage during implantation.

As best shown in FIG. 4, the occipital plate 220 also includes a bonecontacting surface 222 and an opposed surface 224 for seating a spinalfixation element. As will be appreciated by a person skilled in the art,the bone contacting surface 222 is configured to engage the occiput andcan have a variety of configurations. For example, though the bonecontacting surface 222 depicted in FIG. 4 presents a generally convexsurface to the occiput, the bone-contacting surface 222 can be designedto have a shape that maximizes contact between the bone-contactingsurface 222 and the occiput when implanted at a desired implantationsite. Additionally, or in the alternative, the bone contacting surface222 can include various surface features to aid engagement of theoccipital plate 220 with the occiput. By way of example, the bonecontacting surface 222 can include projections that are configured topierce the occiput to help retain the occipital plate 220 at a desiredimplantation site until the occipital plate 220 is anchored thereto, aswill be discussed in detail below.

Further, with reference now to FIG. 6, in one exemplary embodiment of anoccipital plate 620, the occipital plate 620 need not be symmetricalabout the longitudinal axis (L). For example, as shown in FIG. 6, thesuperior most end 630 b of the occipital plate 620 can be offset fromthe longitudinal axis, thereby allowing a surgeon to select an occipitalplate having a shape that best conforms to the desired implantationsite. By way of example, a surgeon could opt to use the occipital plate620 rather than the occipital plate 220 depicted in FIGS. 2-4 to ensureclose contact between the bone contacting surface 622 and the occiputand/or to avoid a protuberance or other such surface feature of theocciput that would prevent the superior end 230 b of the occipital plate220 from laying flush against the bone surface on the midline.

As noted above, the bone-contacting surface can also be contoured so asto substantially conform to a patient's particular anatomical featuresat the desired implantation site. By way of example and with referencenow to FIG. 7, the bone-contacting surface 622 of the occipital plate620 can present a substantially concave surface to the occiput toaccommodate surface features of the occiput (e.g., the median nuchalcrest).

Although the occipital plates described herein can be generally rigidand/or planar, it should be appreciated that the occipital plate 220 canbe configured to allow a surgeon to adapt the bone-contacting surface222 to the target implantation site. For example, the occipital plate220 can be formed of a flexible or malleable material thereby allowingthe occipital plate 220 to conform to the target implantation site. Inother embodiments, the occipital plate 220 can include one or more bendzones formed therein to allow the occipital plate 220 to conform theplate to a surface of the target anatomical location. By way ofnon-limiting example, the bend zones can be formed from channels thatpartially extend between the bone-contacting surface 222 and the opposedsurface 224. Those skilled in the art will appreciate that a variety ofother techniques can be used to provide bendable movement of one or moreportions of the occipital plate 220.

Again referring to the exemplary embodiment depicted in FIGS. 2-4, theopposed surface 224 of the occipital plate 220 is generally configuredto seat a spinal fixation element and can also have a variety ofconfigurations. By way of example, the opposed surface 224 includes agroove 232 that is configured to seat a spinal fixation element therein.The groove 232 extends superiorly along the opposed surface 224 from theinterior end 230 a and can be shaped so as to substantially correspondto the cross-section of the spinal fixation element. Though the groove232 depicted in FIG. 4 includes a semi-circular surface configured tomatch the outer surface of a spinal fixation element having a circularcross-section, the groove 232 can alternatively be shaped so as to matchthe outer surface of a spinal fixation element having othercross-sectional shapes. That is, the shape of the groove 232 can beselected to accommodate a spinal fixation element having, for example,an irregular or rectangular cross-section.

As noted above, the occipital plate 220 can also include any number(e.g., 1, 2, 3, 4, 5, etc.) of openings configured to receive acorresponding number of bone screws (not shown) or any other type ofsuitable anchoring devices for anchoring the occipital plate 220 to theunderlying bone. For example, in the exemplary embodiment of FIGS. 2-4,the occipital plate 220 includes three such openings 226 a-c. As will beappreciated by those skilled in the art, the openings 226 a-c can be ofany shape (e.g., circular, oval, etc.) and/or diameter capable ofsecurely receiving a bone screw or other suitable anchoring device.Additionally, each of the openings 226 a-c can be substantially similarin shape (as shown) or they can each have a distinct shape and/ordiameter. In one embodiment, the openings can have a keyholeconfiguration to enable a suitable anchoring device (e.g., a threadedpost) to be implanted within the occiput prior to positioning theoccipital plate 220 on the occiput. In this manner, the occipital plate220 (with or without the fixation element and floating nut engagedtherewith) can be inserted over a portion (e.g., head) of thepre-implanted anchoring device and manipulated to securely engage thekeyhole opening(s) to fix the occipital plate 220 to the occiput.

With reference now to FIG. 8, in one exemplary embodiment of anoccipital plate 620, the openings 626 a-c can be configured so as topromote an angular displacement of anchor members disposed therethrough.That is, the openings 626 a-c can be configured such that an anchormember disposed therethrough is not directed substantially perpendicularto the plane of the bone-contacting surface 622. For example, as shownin FIG. 8, the lateral edge of openings 626 a-c can be beveled to enablean anchor member to extend laterally away from the occipital plate 620.As will be appreciated by a person skilled in the art, the shape of theopenings 626 a-c can be designed such that a bone screw disposed throughthe openings 626 a-c is directed to an area of the occiput havingsufficient bone density for anchoring the occipital plate thereto.

Further, the alignment and/or positioning of the openings 226 a-c canalso be optimized to conform to the desired anatomical location. Forexample, the location of the openings 226 a-c can be selected such thata bone screw disposed through the openings 226 a-c is directed to anarea of the occiput having sufficient bone density for anchoring theoccipital plate thereto. As shown in FIG. 2, for example, each of theopenings 226 a-c extend through the groove 232 and are substantiallyaligned with one another along the longitudinal axis (L) of theoccipital plate 220. In other embodiments, however, at least one of theopenings 226 a-c can be offset (e.g., staggered) relative to the others.

The openings 226 a-c, however, need not be aligned with the centralaxis. With reference now to FIG. 6, in one exemplary embodiment of anoccipital plate 620, each of the openings 626 a-c instead extend throughthe bone-contacting surface 622 and the opposed surface 624 offset from(e.g., lateral to) the longitudinal axis (L) of the occipital plate 620.Accordingly, the openings 626 a-c extend through the occipital plate 620adjacent the groove 632 rather than through the groove 232 as depictedin FIGS. 2-4. Thus, though the longitudinal axis and the groove 632 ofthe occipital plate 620 can be aligned with the midline of the patient'sspine, the offset openings 626 a-c can enable anchors to be implanted inthe occiput at locations lateral to the midline, for example, to avoiddiseased bone or bone of insufficient thickness or density.

As indicated above, the occipital plate is configured to securely engagea spinal fixation element disposed on the midline of a patient's spine.It should be appreciated that a variety of engagement mechanisms knownin the art can be used to secure a spinal fixation element to theoccipital plate. By way of example, the occipital plate 220 can beconfigured to receive a set screw (e.g., a dual innie) effective tosecure a spinal fixation element to the plate. With specific referencenow to FIGS. 2-4, in an exemplary embodiment, the occipital plate 220includes a pair of flanges 234 that extends from the opposed surface224. The opposed flanges 234 can have a variety of configurations, butgenerally are configured to cooperate with a floating nut 240 to securethe spinal fixation element between the floating nut 240 and the opposedsurface 224 of the occipital plate 220.

As best viewed in FIG. 4, the flanges 234 extend from the lateral edgesof the opposed surface 224 posteriorly and centrally toward thelongitudinal axis (L) of the occipital plate 220, thereby forming acavity between an inner surface of the flanges 234 and the opposedsurface 224. As shown, the terminal ends of the flanges 234 extendsubstantially parallel to the opposed surface 224 and include aprotrusion 236 which extends toward the opposed surface 224.

The floating nut 240 can be configured to engage the flanges 234 of theoccipital plate 220 and can also have a variety of configurations. Inthe exemplary embodiment depicted in FIG. 4, the floating nut 240 caninclude a central portion 244 and lateral projections 246 which extendlaterally away from the central portion 244. The lateral projections 246can additionally include protrusions 248 that are configured to matewith and slidably engage the protrusions 236 of the flanges 234 in adove-tailed manner. In some embodiments, the lateral projections 246 canadditionally include engaging teeth (not shown) which can be configuredto engage reciprocal features formed on the flanges 234 of the occipitalplate 220. In this manner, the engaging teeth and reciprocal featurescan act as a ratchet to inhibit disengagement and/or removal of thefloating nut 240 from the occipital plate 220.

The central portion 244 of the floating nut 240 can also include aposterior surface 250 and an anterior surface 252, at least a portion ofwhich is configured to seat a spinal fixation element. By way ofexample, the anterior surface 252 includes a channel 254 that isconfigured to be disposed in facing relationship with the groove 232formed in the opposed surface 224 when the floating nut 240 engages theflanges 234. As discussed above, though the channel 254 is depicted ashaving a semi-circular surface that is configured to seat a spinalfixation element having a circular cross-section, the channel 254 can beshaped so as to correspond to spinal fixation element having othercross-sectional shapes (e.g., rectangular, irregular).

As best shown in FIG. 5, the central portion 244 of the floating nut 240can also include a bore 256 extending from the posterior surface 250 tothe anterior surface 252. The bore 256 can have a variety ofconfigurations, but generally is configured to allow a locking element242 disposed therein to engage a spinal fixation element disposed in thechannel 254 formed in the anterior surface 252. By way of example, thelocking element 242 can be advanced (e.g., threaded) within the bore 256to secure the floating nut 240 to the spinal fixation element.Accordingly, through the cooperation of the lateral projections 246 ofthe floating nut 240 with the flanges 234 of the occipital plate 240 andthe engagement of the spinal fixation element with the floating nut 240,the floating nut 240 can be positioned in a plane above a plane of theopposed surface 224 such that the spinal fixation element is securelyengaged between the floating nut 240 and the opposed surface 224 of theoccipital plate 220. Movement of the floating nut 240 (and the spinalfixation element) away from the opposed surface 224 and/or movement ofthe floating nut 224 along the central axis (C) of the occipital plate220 can thus be prevented.

In use, the occipital plate 220 depicted in FIGS. 2-5 can be fixed tothe occiput with one or more anchor elements. Further, a spinal fixationelement can be mounted upon the occipital plate 220 (e.g., seated in thegroove 232) such that the spinal fixation element is positioned over amidline defined by the spinal column of the subject, and over any anchormembers that are positioned along the longitudinal axis (L) of theoccipital plate. The spinal fixation element can then be secured to theoccipital plate 220 with the floating nut 240 such that the spinalfixation element is positioned between the occipital plate 220 and thefloating nut 240.

As discussed above, the occipital plate 220 can be fixed to the occiputusing various anchor members known in the art. By way of example, theoccipital plate 220 can be fixed to the occiput by inserting an anchorelement through at least one of the openings 226 a-c extending throughthe bone contacting surface 222 and the opposed surface 224 of theoccipital plate 220. Further, the anchor elements can be inserted intothe occiput at a variety of locations, depending, for example, on thepatient's anatomy. By way of example, the surgeon can fix an occipitalplate 220 to the occiput on the midline via one or more anchor members,and the spinal fixation element can be positioned on the occipital plate220 thereover. Alternatively, an anchor element can be inserted throughthe occipital plate 220 offset from the midline, for example, to avoiddiseased bone or a particularly prominent feature of the patient'socciput.

The method of implanting the occipital plate 220 can also includesliding the floating nut 240 along the spinal fixation element mountedon the occipital plate 220 to position the floating nut 240 between thespinal fixation element and a portion of the occipital plate 220. Forexample, the spinal fixation element can be seated within the channel254 such that the floating nut 240 can be slid therealong such that thelateral projections 246 of the floating nut 240 engage the flanges 234of the occipital plate 220.

As indicated above, spinal fixation assemblies for use in the systemsand methods described herein can have a variety of configurations butare generally configured to secure a spinal fixation element on themidline of a patient's spine. Referring now to FIGS. 9-15, exemplaryembodiments of a spinal fixation assembly are shown in further detail.As will be appreciated by a person skilled in the art, a variety ofprior art spinal fixation assemblies can be modified in light of theteachings herein for use in conjunction with other portions of thesystem 100 of FIG. 1 for positioning the spinal fixation elements 110 onthe midline of the spine.

With specific reference to FIGS. 9-11, one embodiment of a spinalfixation assembly 960 is shown. The spinal fixation assembly 960includes a housing 962 that is configured to seat a spinal fixationelement (e.g., a rod). The housing 962 can be configured in virtuallyany manner capable of receiving and securing the spinal fixation elementtherein. In the exemplary embodiment depicted in FIG. 9, the housing 962includes a base portion 972 and a pair of arms 964 a,b extendingposteriorly therefrom. The arms 964 a,b can have a variety ofconfigurations but generally define a slot 966 (e.g., a U-shapedopening) which extends along the longitudinal axis (L) of the housing.Thus, as described otherwise herein, the housing 962 can be positionedrelative to the spine such that the arms 964 a,b are disposed on opposedsides of the midline and the slot 966 and longitudinal axis (L) arealigned with the midline of the spine. A central axis (C) can also bedefined by the housing 962, the central axis (C) being perpendicular tothe longitudinal axis (L) and extending through the base 972 and theslot 966.

As will be discussed in more detail below with reference to FIG. 11, atleast a portion of the arm 964 a can be configured to contact bone. Forexample, the arm 964 a can include a bone-contacting surface 974 havinga profile configured to correspond to the surface of a lamina 906 thathas been prepared for implantation of the spinal fixation assembly 960.Accordingly, when an anchor member 968 is fully implanted in the lamina906, the bone-contacting surface 974 can sit on the lamina 906 tostabilize the spinal fixation assembly 960 relative thereto. Similarly,the base 972 can include a bone-contacting surface 976 adjacent ananchor-receiving opening 978 that can also be configured to sit on thelamina 906 when an anchor member 960 is fully implanted therein.

The housing 962 can also seat at least one bone anchor member that isconfigured to securely engage a vertebra. In the embodiment depicted inFIG. 10, the housing 962 is configured to receive a single anchor member968. Any suitable type of anchoring device (e.g., plates, hooks, bolts,wires, screws, etc.) can be used to anchor the housing 962 to thevertebra. By way of example, the housing 962 can be configured toreceive a hook that securely engages a portion (e.g., lamina(e), spinousprocess) of a vertebra, without necessarily penetrating the vertebralbone. As shown in FIG. 10, the anchor member can be a bone screw 168having a proximal head 968 p and a threaded distal shank 968 dconfigured to be implanted within a portion of the vertebra. While thebone anchor 968 can have a wide range of sizes and/or shapes, asindicated above, an advantage of trans-lamina delivery can be theability to utilize larger bone anchors 968 as compared to thetraditional lateral mass approach.

As will be appreciated by a person skilled in the art, the bone anchor968 can be securely seated within the housing a variety of mechanisms.For example, in the depicted embodiment, the housing 962 includes ananchor-receiving opening 978 formed in one of the arms 964 a. Theanchor-receiving opening 978 can be disposed through various portions ofthe arm 964 a, but in the embodiment shown in FIG. 9, theanchor-receiving opening 978 extends between the slot 966 and abone-contacting surface 974 of the arm 964 a adjacent the base 972 suchthat the anchor-receiving opening 978 is offset from the central axis(C). In the embodiment depicted in FIG. 10, the anchor screw 968 isseated within the housing 962 such that the distal shank 968 d of theanchor screw 968 extends through the anchor-receiving opening 978inferiorly and laterally away from the housing 962.

Additionally, the anchor-receiving opening 978 can also be configuredsuch that an anchor member is retained in the housing 962. By way ofexample, the anchor-receiving opening 978 can have a minimum diameterthat is greater than the maximum diameter of the shaft 968 d and lessthan a maximum diameter of the head 968 p such that the anchor receivingopening 978 can be effective to retain the head 968 p of the anchormember 968 within the housing 962 while allowing the shaft 968 d toextend therefrom.

The anchor-receiving opening 978 can also be configured such that theanchor member extends therefrom with either a fixed or adjustableorientation relative to the housing 962. By way of example, in oneembodiment, the internal surface 980 of the housing 962 which definesthe anchor-receiving opening 978 can be disposed at a selected anglerelative to the central axis (C) and can be sized or configured suchthat the distal shaft 968 d of the anchor member 968 necessarily extendsthrough the anchor-receiving opening 978 and from the housing 962 at theselected angle. Alternatively, the internal surface 980 of the housing962 can be configured to seat the proximal head 968 p of an anchormember 968 so as to allow the angular orientation of the distal anchor968 d to be adjusted relative to the housing 962. For example, as shownin FIG. 10, the internal surface 980 of the housing 962 can beconfigured to allow for polyaxial movement of the anchor member 968engaged therewith. By way of example, the internal surface 980 of thehousing 962 be substantially spherical so as to correspond with aspherical head 968 p of the anchor member 968, such that the head 968 pcan rotate relative to the housing 962 as in a ball-and-socket joint. Inthis manner, the angle at which the anchor member 968 extends from thehousing can be altered based on the particular anatomy at a desiredimplantation site.

In one embodiment, the arms 964 a,b can additionally include featuresthat provide access to the slot 966 and/or the anchor-receiving opening978. For example, as shown in FIGS. 9 and 10, the arm 964 b disposed onthe opposed side of the central axis (C) relative to the arm 964 a caninclude a window 982 formed therethrough. The window 982 can extendbetween a lateral surface of the arm 964 b and the slot 966 and can beconfigured to provide access to various instruments (e.g., driver,drill, etc.). By way of example, the window 982 can provide access formodifying a bone surface through the anchor-receiving opening 978 (e.g.,forming a bore at a desired implantation site) or for manipulating ananchor member 968 disposed through the anchor-receiving opening 978.

As will be appreciated by a person skilled in the art, the housing 962can also include an engagement mechanism for securing a spinal fixationelement within the slot 966. As shown in FIG. 9, for example, the arms964 a,b can include internal threads 984 configured to receive a closuremechanism (e.g., a set screw) to thereby secure a spinal fixationelement disposed within the slot 966 to the housing 962.

Referring now to FIG. 12-15, another embodiment of a spinal fixationassembly 1260 is shown. Similar to the spinal fixation assembly 960described above with reference to FIGS. 9-11, the spinal fixationassembly 1260 includes a housing 1262 that is configured to seat aspinal fixation element 1210 within a slot 1266 formed between a pair ofarms 1264 a,b which extend from a base 1272. The spinal fixationassembly 1260 depicted in FIGS. 12-14 differs however, in that thehousing 1262 is configured to seat two anchor members 1268 a,b offsetfrom the central axis (C). As best viewed in FIG. 14, the two anchormembers 1268 a,b can extend laterally away from the housing 1262 indifferent directions. Such a configuration, for example, allows thehousing 1262 to be positioned on the midline of a patient's spine and tobe coupled to both laminae of a vertebra.

As best viewed in FIG. 12, the housing 1262 includes twoanchor-receiving openings 1278 a,b formed in the arms 1264 a,b andextending between the slot 1266 and bone-contacting surfaces 1274 a,b.Unlike the spinal fixation assembly 960 depicted in FIG. 9, however, theinternal surface 1280 a,b of the housing 1262 which defines theanchor-receiving openings 1278 a,b does not fully enclose theanchor-receiving openings 1278 a,b. That is, the arms 1264 a,b extendfrom only a single “closed” side of the housing 1262. As will bediscussed in detail below, the “open” side of the housing 1262 thereforecan allow the housing 1262 to receive proximal heads 1268 p of boneanchors 1268 that have been pre-installed in vertebral bone.

Further, though the arms 1264 a,b of the spinal fixation assembly 1260are shown without a window 982 as depicted in FIGS. 9-11, one of skillin the art will understand that the arms 1264 a,b can additionallyinclude windows to provide access to the heads 1268 p of the boneanchors 1268 a,b seated within the anchor-receiving openings 1278 a,b.

With specific reference now to FIG. 14, an assembled spinal fixationsystem 1260 is shown in which a spinal fixation element 1210 is disposedwithin the slot 1266 of the spinal fixation assembly 1260. As discussedabove, the spinal fixation element 1210 can be retained within the slot1266 by the engagement of the set screw 1270 with internal threads 1284formed in the arms 1264 a,b. Advancement of the set screw 1270 towardthe base 1272 can be effective to similarly displace the fixationelement 1210 toward the base 1272. This displacement of the fixationelement 1210 can force the heads 1268 p of the anchor members into closeengagement with the internal surfaces 1280 a,b. As will be appreciatedby a person skilled in the art, the engagement of the spinal fixationelement 1210 with the anchor members 1268 a,b can therefore help retainthe anchor member 1268 within the anchor-receiving openings 978 a,b.

As indicated above, spinal fixation elements can additionally includefeatures to aid in the integration of the spinal stabilization systemand promote post-surgical recovery. For example, as shown in FIG. 14,the spinal fixation element 1210 includes one or more fin(s) 1286 havinga plurality of thru-holes 1288 formed therethrough. As will beappreciated by a person skilled in the art, the fin 1286 can be integralwith (e.g., formed on) the spinal fixation element 1210 or can beremovably coupled thereto, either before or after the spinal fixationelement 1210 is seated within the slot 1266 of the spinal fixationassembly 1260. The thru-holes 1288 can provide an attachment point formuscles and other soft tissue that was previously connected to a spinousprocess, for example, that was resected during the spinal stabilizationprocedure.

The spinal fixation assemblies described above with FIGS. 9-15 can beutilized in spinal stabilization techniques. The fixation assembliesdescribed above can be positioned within any number and/or type (e.g.,cervical, thoracic, lumbar) of vertebra as required by any givenprocedure. Also, the method can include positioning the fixationassemblies in various manners so as to optimize the orientation of thefixation element relative to the patient's spinal column. For example,in one embodiment, the slots of the various fixation assemblies can bepositioned on the midline of a patient's spine such that a singlefixation element is seated and secured within the slots on the midlineof the patient's spine.

Such methods can include positioning a first fixation assembly upon afirst vertebra, the first fixation assembly having a proximal housinghaving a base and a pair of arms. The first fixation assembly can bepositioned such that a slot extending between the arms and along alongitudinal axis of the first fixation assembly is aligned with amidline of the spine. An anchor member, seated in the first fixationassembly, can extend away from the housing at an acute angle relative toa central axis that is generally perpendicular to the longitudinal axis.A second fixation assembly, that is the same or different from thefirst, can be positioned within the second vertebra such that a slot ofthe second fixation assembly is aligned with the slot of the firstfixation assembly. A spinal fixation element is then positioned withinthe slots of the first and second fixation assemblies such that thespinal fixation element extends along a midline of the spine. Finally,the spinal fixation element is secured within the slots of the first andsecond fixation assemblies.

As discussed above, the first and second fixation assemblies can besecured to various portions of the vertebrae. For example, during thestep of positioning a first fixation assembly, the anchor member can besecured to a lamina of the first vertebra in a trans-lamina orientation.Additionally, the various portions of the target vertebrae may bemodified or truncated so as to further optimize the procedure and/orprovide a desired clinical outcome (e.g., decompression to alleviatepressure on the spinal cord). As discussed above, by removing portionsof the target vertebra, the spinal fixation assemblies can access anoptimal entry point of the vertebral bone. Further, the bone-contactingsurfaces of the housing can be shaped so as to substantially correspondto the portion of bone to which the spinal assembly is implanted toimprove the engagement therebetween. For example, in the vertebra 904depicted in FIG. 11, the spinous process and one of the laminae (to theright in FIG. 11) has been removed such that the fixation assembly 960is secured to the vertebra 904 by a single anchor 968 implanted in theremaining lamina 906 in a trans-lamina orientation. In one embodiment,the anchor member 968 is seated within the housing 962 before beingsecured to the lamina 906. By way of example, a driver can be insertedthrough the window 982 to drive the anchor member 968 into the lamina906.

With reference now to FIG. 15, a vertebra 1204 is shown in which thespinous process has been removed. The fixation assembly 1260 is securedto the laminae 1206 via two anchor members 1268 implanted within eachlamina 1206 in a trans-lamina orientation. As indicated above, theanchor members 1268 can be pre-installed in the lamina and the fixationassembly 1260 can subsequently receive the heads 1268 p of the anchormembers 1268. For example, as shown in FIG. 14, the “open” side of thehousing 1262 can be pressed in a caudal direction over the heads 1268 pof the anchor members 1268 (e.g., snapped onto the heads 1268 p) afterthe anchor members 1268 have been implanted in the laminae 1206.

One skilled in the art will appreciate further features and advantagesof the presently disclosed method and system based on theabove-described embodiments. Accordingly, the present disclosure is notto be limited by what has been particularly shown and described, exceptas indicated by the appended claims. All publications and referencescited herein are expressly incorporated herein by reference in theirentirety.

What is claimed is:
 1. A spinal fixation assembly, comprising: a housinghaving an anterior base and a pair of arms extending posteriorlytherefrom, the pair of arms defining a slot therebetween configured toseat a spinal fixation element, the slot extending along a longitudinalaxis of the housing and configured to be aligned with a midline of thespine; the housing having a central axis perpendicular to thelongitudinal axis and extending through the base and the slot; ananchor-receiving opening formed in one of the pair of arms and extendingbetween the slot and a concave bone contacting surface of the armadjacent the base; and an anchor member having a head positionablewithin the anchor-receiving opening and a shank configured to engagebone; wherein the anchor-receiving opening is disposed entirely on oneside of the central axis and configured such that when the head of theanchor member is completely disposed within the housing, the head isdisposed entirely on one side of the central axis and the shank extendsanteriorly and laterally away from the housing at an acute anglerelative to the central axis.
 2. The assembly of claim 1, wherein theanchor-receiving opening is defined by an internal surface of thehousing.
 3. The assembly of claim 2, wherein the internal surface of thehousing is configured to engage a head of the anchor member.
 4. Theassembly of claim 3, wherein the internal surface of the housing issubstantially spherical to allow for polyaxial movement of the anchormember.
 5. The assembly of claim 1, wherein the base has a bonecontacting surface adjacent the anchor-receiving opening.
 6. Theassembly of claim 5, wherein the bone contacting surfaces of the arm andthe base are configured to sit on the lamina.
 7. The assembly of claim1, wherein the pair of arms comprise a first arm and a second armdisposed on opposed sides of the central axis relative to one another.8. The assembly of claim 7, wherein the second arm includes a windowformed therethrough, the window being configured to provide access to adriver for manipulating an anchor member disposed through the anchorreceiving opening in the first arm.
 9. The assembly of claim 1, whereinthe anchor-receiving opening is angled relative to the central axis topermit the anchor member to be implanted within the vertebra in atrans-lamina orientation.
 10. The system of claim 1, wherein, when thehead of the anchor member is completely disposed within the housing anda spinal fixation element is locked within the housing, a portion of thehead of the anchor member lies flush with the slot and the locked spinalfixation element applies a force to the anchor member that fixes aposition of the anchor member with respect to the housing.
 11. A spinalfixation system, comprising: a housing having a base and first andsecond arms extending therefrom, the first and second arms defining aslot therebetween, the slot extending along a longitudinal axis of thehousing and being configured to be aligned with a midline of a subject'sspine, the housing having a central axis perpendicular to thelongitudinal axis and extending through the base and the slot; a spinalfixation element configured to be disposed in the slot such that thespinal fixation element extends along the midline of the subject'sspine; an anchor member configured to be disposed through ananchor-receiving opening extending through the first arm between theslot and a bone contacting surface of the first arm; and a closuremechanism configured to threadably engage the first and second arms forsecuring the spinal fixation element within the slot; wherein, when theclosure mechanism threadably engages the first and second arms such thatthe spinal fixation element and the head of the anchor member aresecured within the housing, and the head of the anchor member iscompletely disposed within the housing, the central axis extends througha center of the closure mechanism and through the spinal fixationelement, the head of the anchor member is disposed entirely on one sideof the central axis, and a shank of the anchor member extends away fromthe housing at an acute angle relative to the central axis.
 12. Thesystem of claim 11, wherein the anchor member comprises at least one ofa screw configured to be imbedded in a lamina and a hook configured tobe hooked onto a lamina.
 13. The system of claim 11, further comprisinga fin coupled to the spinal fixation element, the fin having one or moreholes to which tissue can be attached.
 14. The system of claim 11,wherein inner surfaces of the first and second arms have threads forthreadably engaging the closure mechanism.
 15. The system of claim 11,wherein, when the closure element threadably engages the first andsecond arms and locks a spinal fixation element disposed in the slot,the spinal fixation element applies a force to the anchor member andfixes a position of the anchor member with respect to the housing.